The present invention relates to devices for detecting contamination of the vacuum in a closed vessel. In particular, the present invention relates to an apparatus for indicating vacuum degradation in high voltage circuit current interrupters.
High voltage vacuum circuit breakers are employed in electrical distribution systems to interrupt current under fault conditions. Vacuum conditions are employed in the vicinity of the contacting electrodes to further enhance the ability of these switch devices to interrupt high voltage currents. However, these circuit breaker devices are employed under field conditions and are required to sustain a vacuum, typically less than 0.01 torr, for extended periods of time, typically in the range of 20 years or more. Under these conditions and over this period of time, it is possible that a small number of these switches could develop air leaks, thereby degrading the vacuum condition and rendering the device incapable of proper interruption of some high voltage current conditions. Accordingly, it is desired to be able to monitor these switches in an efficient, reliable and economical manner to ensure that loss of vacuum has not occurred. Any instrumentation device monitoring the vacuum condition must be simple and reliable and must operate so that the performance of the circuit breaker is not affected by the presence of the monitoring device. Additionally, the monitor must be reasonably low in cost and must be capable of determining vacuum conditions during long quiescent periods when the circuit breaker operates in a normal, current-conducting mode with the contacts closed.
It is known by experts in the vacuum tube arts that relatively large leaks in vacuum envelopes may be detected by visually observing changes in the appearance of a barium getter film deposited on the glass wall of the vacuum envelope. However, this method of vacuum monitoring is not useful for observing the operation of vacuum switches because such switches are normally completely enclosed in metal with dry air or SF.sub.6 insulating the switch from the surrounding metal enclosure. Accordingly, in such current interrupters, the vacuum switch is not visible. Additionally, many of these circuit interrupters are mounted at locations fairly high off the ground. Accordingly, climbing to their level would be both awkward and dangerous while the line is energized. It is further noted that it is generally under conditions when the line is energized that one would most likely wish to determine the state of the vacuum conditions within the interrupter. Furthermore, high voltage vacuum switches do not employ barium getters because barium is a low work function material and if it were to become deposited on the electrodes, it would cause a large loss in voltage stand-off ability. Accordingly, for this reason it has therefore been appreciated that the use of barium in high voltage vacuum interrupters is generally inappropriate.
In U.S. Pat. No. 4,021,702, issued May 3, 1971 to Manfred Rimmrott, it is also observed that it is desired to detect the presence of a degraded vacuum condition in electrical vacuum interruption devices. In this invention, it appears that the infrared radiation eminating from the hot electrodes within the vacuum switch is monitored. However, the apparatus in this patent is only usable as the switch is opened. It provides no warning of loss of vacuum during quiescent switch conditions. It is noted that for the circuit interrupter switch as contemplated herein that these switches may, in fact, close for many months before fault conditions arise to trigger their actuation.
In U.S. Pat. No. 3,594,754, issued July 20, 1971, to Roy E. Voshall, there is also apparently disclosed an apparatus for detecting loss of vacuum. However, in this apparatus, a voltage is applied between one electrode and a floating shield with detection being based upon current flow to the shield.
The above-described effects concerning barium films are described, for example, in Mitsubishi publication number L-32645A-IT 6901. It should furthermore be pointed out that any instrumentation employed in the detection and monitoring of vacuum conditions in high voltage circuit interrupters must also achieve isolation between the ground and the high voltage circuit so that individual monitoring the vacuum conditions are not exposed to high voltage conditions.